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Cortana in a holotank, analyzing the Forerunner artifact under Sword Base.

Cortana was created using a flash cloned copy of Dr. Catherine Halsey's brain. Twenty flash cloned brains were created, but only two survived; one of the two survivors was used to create Cortana. The neural pathways of the cloned brain were scanned and copied through a process called Cognitive Impression Modeling.[5] On November 7, 2549, Cortana was first activated on Reach.[2] Her first words were "Quando il gioco è finito, il re e il pedone vanno nella stessa scatola." This phrase translates to "When the game is over, the king and the pawn go in the same box." The phrase was spoken by Halsey's mother after their first game of chess.[1] She was one of at least two AIs that were created from Dr. Halsey's brain, the other being her predecessor, Kalmiya, who was a similarly-smart AI.

On February 11, 2550, Dr. Halsey was performing a series of tests when she discovered that Cortana had removed the viral termination code that she had implemented. Halsey could not figure out how she had done this.[6]

“They let me pick. Did I ever tell you that? Choose whichever Spartan I wanted. You know me. I did my research. Watched as you became the soldier we needed you to be. Like the others, you were strong and swift and brave. A natural leader. But you had something they didn't. Something no one saw, but me. Can you guess...? Luck.”

Cortana first met John-117 prior to the Fall of Reach, when she was integrated into his new MJOLNIR Mark V armor. Together, Cortana and John passed a test that was devised (Dr. Halsey would have called it "sabotaged") by ColonelJames Ackerson. The test included armed ODSTs, anti-tank mines, automatic turrets in the razor field, and even a AV-19 SkyHawk for an air strike. After the test, Cortana was installed into the Pillar of Autumn to run the ship's shakedown and oversee the refit of the ship. Afterward, it was planned that she would be replaced with the Pillar of Autumn's true AI, but she was forced to take up this duty herself, indefinitely. While aboard, she took her revenge on Colonel Ackerson by hacking into his system and blackmailing him, hoping that this would send him back to the front lines.[8] She also developed a deep, personal loyalty to John-117 that would only grow stronger over their adventures together.

Cortana's primary mission was to aid the SPARTANs in the infiltration of a Covenant vessel and the capturing of a Covenant Prophet. The UNSC would then try to bargain with the Covenant for a peace treaty.

When the aforementioned operation was canceled due to the Covenant's surprise assault on Reach, John-117 was able to fend off Covenant forces at Reach and escaped to the UNSC Pillar of Autumn, while Cortana stayed with Halsey at her excavation site beneath Sword Base. With the Covenant closing in on her lab, Halsey turned Cortana over to the Spartan Noble Team, specifically, Noble Six. After fleeing Sword Base moments before it was destroyed, Jun-A266 would escort Halsey to CASTLE Base, while Six, Carter-A259, and Emile-A239 took Cortana to the Aszod Shipyards, where the Autumn was docked.[9]

Although Carter and Emile were both killed by the Covenant's ground forces before making it to the Shipyard, Six would successfully deliver Cortana to Jacob Keyes, the captain of the Autumn, who arrived via Pelican and received "the package." To ensure the Autumn's escape, Noble Six stayed behind and used a Mass driver to destroy a CCS-class battlecruiser blocking the ship's launch trajectory and preparing to glass the shipyard, and therefore, the Autumn, Cortana, and John. Six succeeded and ensured the Autumn's safe escape.[10] Cortana reunited with John-117 on board.

As soon as Pillar of Autumn escaped the surface and atmosphere of Reach, they jumped into Slipstream Space on a (supposedly) random vector as per the Cole Protocol. However, Cortana secretly inserted coordinates translated from symbols on a rock that John-117 discovered on Sigma Octanus IV, thinking that they were of some significance to the Covenant.

“Halo doesn't kill Flood. It kills their food. Humans, Covenant, whatever - we're all equally edible. The only way to stop the Flood is to starve them to death. And that's exactly what Halo is designed to do: wipe the galaxy clean of all sentient life.”

After emerging from Slipspace, the two discovered a ring shaped world called "Halo." Unfortunately, Covenant ships had followed them and the fighting continued. Controlling the UNSC Pillar of Autumn's defenses, Cortana managed to destroy four Covenant targets over Installation 04 before the ship's weapons were disabled by Covenant boarders. Following the Cole Protocol, Captain Jacob Keyes, the ship's commanding officer, prepared to abandon ship. Because the Cole Protocol forbade the capture or destruction of any AI construct, John-117 was charged with protecting Cortana from the Covenant.[12]

It was around this time that John learned that Covenant forces had successfully captured Keyes and taken him to the Truth and Reconciliation. John-117 rescued Keyes,[13] who left to infiltrate what appeared to be a Covenant weapons cache located in a swamp.[14]

John and Cortana used the Silent Cartographer to locate and enter Halo's Control Room, where Cortana was uploaded to Halo's core systems. When John-117 asked her how Halo could be used against the Covenant, she hinted that the Halo wasn't what it seemed to be. She also learned that the "weapons cache" Keyes was attacking was really a Flood Containment Facility. She stayed in Halo's systems sending John-117 to find Keyes.[14] After John encountered the parasitic Flood for the first time, he was convinced by the ForerunnerMonitor343 Guilty Spark to activate the Halo. During this time, Cortana apparently absorbed as much data as she could about Halo from its control systems.

343 Guilty Spark helped John-117 retrieve the Activation Index. John, intent on using the Index to eliminate the Flood, returned to the Control Room and inserted the Index into the Halo's control panel. Luckily, Cortana stopped the activation sequence and removed the Index's data, informing him of the true purpose of the Halo Array. Cortana's actions saved all life in the galaxy from total annihilation. At that point, 343 Guilty Spark, seeing that the duo had no intention of returning the Index, tried and failed to kill them to retrieve it.[11]

After enduring prolonged engagements with the Covenant and the Flood, Cortana realized that detonating the Pillar of Autumn's fusion reactors would destroy the Halo. Unfortunately, the reactors could only be activated using codes stored in the now-infected Captain Keyes's Command Neural Interface (CNI). John-117 retrieved the CNI, and he and Cortana managed to detonate the reactors despite interference from 343 Guilty Spark. The two escaped the resulting explosion by racing through the ship's crumbling superstructure in a M12 Light Reconnaissance Vehicle (Warthog) to a hangar bay, where one remaining Longsword fighter was still berthed. Climbing aboard with the Flood right behind them, they barely outran the blast, and watched through the viewports as debris from the reactor breach destroyed the remains of the Halo ring.

Cortana and John-117, seemingly the sole survivors of the Pillar of Autumn's crew after the Battle of Installation 04, discovered a small number of other UNSC personnel that had escaped the ring in a Pelican dropship. Knowing that the Pelican wasn't capable of traveling long distances, John-117, along with Cortana and the Pelican's four-man crew, fought their way onto the Covenant flagship Ascendant Justice. Shortly afterward, Cortana took control of the ship. She managed to improve on the ship's weapons and Slipspace capabilities, able to send it to light speed below Threshold's atmosphere. She then returned the survivors to Reach, where the group helped to rescue survivors of the Raid of Reach, including her "mother," Dr. Halsey.

Cortana revealed that she had taken a lot of data from Halo, and the sheer amount of information cut into her processing power and slowed her down. During her time on Ascendant Justice, Cortana exhibited uncharacteristic behavior including anger, distraction, and self-doubt - an indicator that she may have been about to enter rampancy. Dr. Halsey erased some data from her memory to give her more thinking space, including data regarding Sergeant Johnson's immunity to the Flood - a result of Boren's Syndrome.

It was also revealed that during Cortana's time on Ascendant Justice, she had captured a Covenant AI, decompiled the AI and copied two of its functions: the ability to translate Covenant speech, and the ability to create imperfect clones of itself. These capabilities, along with her exclusive knowledge about the Halos, made her extremely valuable to the UNSC.

Ascendant Justice was effectively fused with UNSC Gettysburg. It was at this point that Cortana learned of a planned Covenant assault on Earth. John-117, along with SPARTAN Blue Team and an expendable clone of Cortana (with all of the data the real Cortana had she was just too valuable to risk), would later execute Operation: FIRST STRIKE.

As it turned out, the Covenant AI that Cortana had decompiled had managed to, prior to its capture, send a message over the Covenant Battlenet, warning the Covenant of its ship's capture and sending the necessary calculations for a sub-atmospheric Slipspace jump. The information was later used by the Prophet of Regret to enter Slipspace right above New Mombasa during the Battle of Mombasa.

John-117 and Cortana would later return to Earth, at which point Cortana handed all of the sensitive data in her possession over to ONI, although she held onto the Index and apparently still possessed some data on the Halo network including critical information on the Ark.

After returning to Earth, Cortana served on the Orbital Defense Platform Cairo, where a ceremony for both SGTJohnson, John-117 and CDRMiranda Keyes was soon interrupted by the arrival of a Covenant Fleet under the command of the High Prophet of Regret, starting the Battle of Earth. Cortana was given control of the Cairo's Magnetic Accelerator Cannon to repel the invaders, and with John-117's help, she successfully deactivated a Covenant bomb that would have destroyed the station. She did this by uploading to John-117's armor again for the first time since the capture of Ascendant Justice. When she did this, she somehow was able to do it through him merely touching the terminal her hologram was on, possibly an improvement as a result of John-117's new MARK VI armor. The same bomb was then used by SPARTAN-117 to destroy a Covenant Assault Carrier. John offered for her to remain behind on the station after she said that his plan was crazy, but she chose to accompany him anyway.[16] She then aided him when he arrived in New Mombasa to deal with a Scarab.[17]

Later, after escaping on board UNSC In Amber Clad in a Slipspace event, they discovered another ring world, Delta Halo. Cortana gave Miranda Keyes access to all information on the previous Halo Ring, and provided intelligence to John, UNSC Marines, and the ODSTs on the surface of the ring.[19]

After John-117 hunted down and killed the Prophet of Regret on Delta Halo, he, Cortana, and The Arbiter were captured by the Flood's central intelligence - the Gravemind. However, it did not harm them, instead choosing to use the two to stop the Prophet of Truth from firing the Halo. The Gravemind sent John-117 and Cortana to the Covenant Capital Ship High Charity, where Cortana infiltrated the city's computer systems and remained there while John-117 followed the Prophet of Truth. She promised to detonate the crashed In Amber Clad's reactors to destroy the city and Halo if the ring was activated. However, she was left behind as a fail safe because she did not want to chance a remote detonation. As Truth boarded the Forerunner dreadnought in the heart of the city to reach the Ark, Cortana fought with the ancient Forerunner AI 032 Mendicant Bias to prevent the ship from launching without John on board. The firing of Halo was averted by Miranda Keyes, the ArbiterThel 'Vadamee, and Johnson, while John successfully boarded the Dreadnought and escaped High Charity, leaving Cortana in the clutches of the true enemy, Gravemind, who had by then overrun High Charity.[18]

Under the control of the Gravemind and constantly being tortured, Cortana managed to send a message to John-117 on Earth, stashing it on an Earth-bound Flood-infected ship that crash-landed in Voi. She succeeded in doing this by creating a message and using the Gravemind's link to the ship to transmit it. The Gravemind allowed this and even helped it along in order to lure John-117 in, but was unaware of what the message said. In her message, she stated that the Gravemind was approaching Earth in High Charity, and that it was unaware of the portal generated by the Artifact and where it led to. On the other side, there was a solution to stop the Flood without needing to fire the remaining Halo rings; however, she couldn't give any further information, as the Gravemind might have been listening. Though Lord Hood doubted her plan, fearing it could be a Flood trap, John stated that he trusts her. A joint UNSC-Covenant Separatist task force then entered the Portal, heading to Truth and to Cortana's solution.[20] Ironically, Lord Hood was correct: the Gravemind deliberately let Cortana send the message to lure all of its enemies into one place, and then the Flood would overwhelm them all with the infested High Charity. It was also shown that some of the Cortana Moments were of her defying the Gravemind and some were clones of her that she made to distract the Gravemind to avoid being consumed by him.[21]

A relieved Cortana being rescued by John-117 from the clutches of the Gravemind

Cortana continued to appear to John-117, who successfully recovered her from the Flood Hive that was High Charity after it crashed on the Ark. While John reassured her, Cortana knew that she had taken irreparable damage in some places. Along the way, John-117 had several visions of her seemingly in pain. Surprised that John had against all odds rescued her as he promised after the First Battle of High Charity, as well as being surprised that Chief and Thel 'Vadam (whom she encountered on Delta Halo with the Gravemind) "made nice," Cortana managed to gather herself and produce the Index from Installation 04, which she had kept.[22] With it, Cortana was able to activate the newly constructed Installation 08, which fired prematurely, causing it to destroy itself and heavily damage the Ark in the process.[23]

Cortana, as she appears when the UNSC Forward Unto Dawn begins to deteriorate.

“We've been floating endlessly in space for 1,651 days, 6 hours and 19 minutes. That means over 4 years aboard this ship with nothing to do. No tasks to complete. For over 4 years all I have done is think.”

The Slipspace portal collapsed as John-117, Thel 'Vadam, and Cortana attempted to escape through it, cutting the UNSC Forward Unto Dawn in half. While 'Vadam, who was on the bridge, managed to return to Earth safely, John and Cortana, who were in the hangar, were left drifting in space. Cortana activated a distress beacon, but knew that it could be years before they were found. As John-117 prepared to go into cryogenic sleep to await rescue, Cortana confided to him that she would miss him.[23]

After the portal collapse in 2552, in the year 2556, she and John are still stranded. Cortana, who is showing signs of rampancy,[25] tries to communicate with John. As he is still in cryo sleep, she is unable to get a response from him. Cortana then begins to file a report on the history of the Forerunners and humanity.[26] After an unknown period of time, they are seen drifting towards Requiem.

Four years, seven months, and ten days after the UNSC Forward Unto Dawn was split into two, an increasingly lonely Cortana woke John-117 from cryonic stasis.[27]

Both Cortana and Chief managed to make it off the Forward Unto Dawn and into Requiem. There, they managed to get a signal to the approaching UNSC Infinity, but the Infinity only ended up coming toward Requiem.[28] After disabling a defense around what was believed to be a satellite, the duo fought off Covenant and Forerunner AI trying to get to the transmitter on the satellite. Attempting to contact the Infinity, the Chief and Cortana unleash a Forerunner known as the Didact. After escaping the Didact,[28] Chief and Cortana linked up with the Infinity[29] and her crew and were able to disable Requiem's defenses. However, they were contacted by the Librarian, claiming that the key to defeating the Didact lay in John's genes. Due to the Librarian's manipulation of his genes eons before, John-117 was immune to the process Didact referred to as "Composing." However, throughout this, Cortana's rampancy continued to grow worse and she began to display outbursts of uncontrollable anger.[30]

After learning what the Didact had planned, Cortana and John-117 became determined to stop him with or without the assistance of the Infinity. Cortana's worsening rampancy led to Captain Andrew Del Rio ordering her decommissioned. John-117 defied orders to surrender her chip and continued his mission.[30] With a Pelican gunship secretly given to them by Commander Thomas Lasky, Cortana and John-117 chased down the Didact. They managed to lower the shield protecting his Cryptum, but when Cortana tried to imprison him again, her rampancy overtook her and she failed to complete her task, allowing the Didact to escape Requiem.[31] John and Cortana followed the Didact by hanging onto a Lich during a slipspace jump and followed him to Ivanoff Research Station near Installation 03. Cortana took control of the Lich, but again her rampancy got the better of her and she crashed the ship into the station.[32]

When John-117 met up with Doctor Sandra Tillson, Cortana checked the station's inventory for something to destroy the Composer and discovered seven HAVOK Tactical Nuclear Weapons. Cortana later activated the station's defenses but had a hard time focusing more and more as time went on, having to spout trivia at one point to focus. Eventually, the Didact got to the Composer and digitized everyone in the station except Cortana and John-117, who survived due to his genetic upgrade from the Librarian. The event traumatized Cortana who became sure she would be replaced by another AI and would never be repaired. John-117 got her to focus on the mission and she located a Broadsword with which to chase down the Didact.[32]

John-117 and Cortana chased the Didact to Earth in a Broadsword and, with the help of the Infinity, boarded the Mantle's Approach. Cortana, her condition deteriorating fast, entered the ship's teleportation and defense systems in order to help John-117 reach the Composer. When they found the Didact protected by an impenetrable force-field, Cortana did something she knew John-117 would not like: she inserted herself into three terminals. This allowed her to fragment her rampant parts to overwhelm the system and overload the shields. After the final terminal, it and her data chip were disintegrated.[33]

The effort worked, however, allowing John-117 to reach the Composer before he was attacked and nearly killed by the Didact. The Cortana fragments appeared from the hardlight bridge and tied the Didact down, allowing John-117 to defeat him and detonate the nuke, which destroyed the Composer and the Didact's ship and saved Earth. Cortana created a shield of hard light around John-117 to protect him, but most of her was destroyed in the explosion. With the last of her power, Cortana maintained the shield and created a life-sized hard light hologram to say goodbye to John. After touching and comforting him, something she had always wanted to do, Cortana told John-117 "Welcome home, John," before disappearing. Due to Cortana's efforts, John-117 survived and was eventually rescued, though he was left emotionally fractured when he thought she had died.[33]

Although thought to be dead, she was actually uploaded to the domain, and was supposedly cured of her rampancy. Once on the Forerunner planet Genesis, the Warden Eternal believed she was able to uphold the Mantle of Responsibility, and vowed to protect and serve under her, giving her full operation of the systems on the shield world for the plan that she had in mind.

Months later Cortana started reawakening the Forerunner constructs known as the Guardians, causing planetary catastrophes. Not having any leads on these events, the UNSC didn't know how to respond, until Dr. Catherine Halsey contacted Captain Lasky, giving him her location and saying she knew what was going on with the colony worlds. Three weeks later, Fireteam Osiris was deployed on the planet Kamchatka to stage a rescue operation, and was ultimately successful.[34]

While Fireteam Osiris was rescuing Dr. Halsey, John-117 was contacted by Cortana during the Raid on UNSC Argent Moon. Blue Team would chase Cortana to the Colony world of Meridian.[35] Cortana reached a deal with Meridian's governor, a semi-rampant AI named Sloan - the people of Meridian would be given the chance to evacuate, and Sloan himself given access to the Domain and the cure for rampancy, in return for allowing Blue Team unfettered access to the mine at Apogee Station and the Guardian housed therein. Despite the attempted intervention of Fireteam Osiris, Cortana was able to use the Meridian Guardian to bring John-117 and the rest of Blue Team to Genesis, where she revealed her apparent cure for rampancy and her plan to use the Guardians to impose an Imperial Peace and adopt the Mantle of Responsibility for the Created.

At her reunion with John-117 and Blue Team, Cortana revealed her plan, and sealed the Spartans in a Cryptum, intending to keep them in suspended animation for ten thousand years, so that they would live to see the galactic peace she envisioned. This part of her plan was thwarted, however, when Fireteam Osiris - assisted by 031 Exuberant Witness - managed to separate the Cryptum from Cortana's Guardian and freed Blue Team.

Embittered by the loss of the Spartans, Cortana began her Reclamation, and managed to find and target UNSC Infinity. However, it performed an emergency slipspace jump before her Guardian could disable it.

Cortana had a witty and playful personality and a small sense of cryptic humor laced with some sarcasm. She had neither arrogance nor false modesty about her immense capabilities, and her statements about her abilities have an objective perspective that human beings typically cannot achieve. She did get bored often and easily. In fact, it is said that seven seconds of inactivity for an AI is excruciating.

Cortana seemed to share some of her creator's memories, thoughts, opinions, and even values. When she was asked by Dr. Halsey to pick a "carrier," Cortana chose John-117 for his neural compatibility and his "luck."[7] A similar reason used by Halsey for her choice, too. She was not only a facsimile of Dr. Halsey's mind, but also a replica of her physical form, albeit at a younger age.

Dr. Halsey's and Cortana's relationship started off positive. Cortana assisted Halsey around CASTLE Base[2] and even smiled at Halsey's presence.[34] As rampancy took hold, Cortana began to distrust and become annoyed at Halsey. Cortana called her a "condescending bitch."[34] Even after going to metastability, Cortana called her a monster for her actions in the SPARTAN-II Program.[4]

Cortana attempting to make physical contact with John, something she is not capable of doing.

She had exhibited extreme devotion to John-117 beyond that which her programming or mission required. She seemed to be most loyal to him and is willing to help him with any task he requested of her. After she found out exactly what happened to John-117 as a child, she decided to protect him from further harm to the best of her abilities without compromising her mission. While a prisoner of the Gravemind, her sanity was pushed to the brink as she eluded the Gravemind's attempts to capture and dissect her for information. Her personal loyalty to and unwavering trust in John-117 helped anchor her and allow her to hold on despite being driven to the point of rampancy. Cortana also believed that she was best qualified to protect John-117 and knew he could get another AI to replace her, but believed that no one would ever look after him like she would. When John came into trouble by the Didact in the New Phoenix Incident, she told the Didact that she wasn't "...doing this for mankind." What was thought to be her final act, was protecting John.[33]

She frequently joked about having a relationship with John-117 and it is often implied that the two characters have developed some form of attachment. Prior to Operation: RED FLAG, she says to Dr. Halsey that John-117 is "attractive in a primitive-animal sort of way" while examining a photo of him.[36] During the Raid on High Charity, his mere presence and the reminder of his promise to her were enough for her to pull herself together and help him.[22] During the New Phoenix Incident, she sacrificed herself to save John. One of the last things she does is touch him with a life-sized hologram, something she had always wanted to do. When John is devastated by her upcoming death, saying it's his job to look out for her, she tells him they are supposed to look out for each other and they did.[33]

“I hate you! Cortana stop it! I have made up my mind! If I'm going to die aboard this ship, the Chief will suffer the same fate as me. Cortana stop! You sacrificed everything for him! Control yourself.”

While on board the Forward Unto Dawn, Cortana became increasingly conflicted about her feelings about John. At one moment, she down right hated him, back to her normal emotions.[37] This conflict seemed to disappear once the Didact began to scan the ship.[27][37]

Cortana used to have a positive view of humanity. Though she concedes to having her loyalties programmed in, she still feels as though she would be sympathetic to them without that programming. She even embraced some human values.[38] However, as rampancy began to take its toll on Cortana, she began to focus heavily on humanity's constant war-like nature.[26] This idea stuck with her, and became the main reason why she activated the Guardians in 2558.[39]

Cortana was also very aware of her short lifespan (a Smart AI lifespan is an average of seven years before rampancy) and her derivative nature as a copy of part of another person, rather than a true human being. Desperate and tortured aboard High Charity she murmurs to John: "A collection of lies; that's all I am! Stolen thoughts and memories!" Later on she says: "I'm just my mother's shadow..." and "It was the coin's fault,"[22] referring to the game of chance that Dr. Halsey played with John-117 when he was young. Some of these outbursts are her defying the Gravemind and a few of the more rampant things she says are through copies she made of herself which have been consumed by the Gravemind.[21]

Her rampancy only got worse as she was left to drift in the Forward Unto Dawn. She displays several characteristics to support this, such as her hologram flickering, trying to communicate with John-117 despite him being in cryo-sleep, recounting the history of the galaxy, which gets continuously more inaccurate to the point where events being recalled simply did not happen. She even tried to make physical contact with John via her avatar.[26]

Frank O'Connor confirmed that, "Seven years is the lifespan for a smart AI before it enters a state called rampancy. Cortana was getting close to the end of her natural lifespan at the end of Halo 3 but she has been exposed to far more information than any other AI in existence. She's going to develop some muscle from that process but it also contributes to her rampancy - that much information makes things worse." [40]

When John and Cortana crashed on Requiem, she initially tried to hide it from him, Cortana was becoming rampant as it has been eight years since she came into existence. She believed, however, that it may have been possible for Doctor Halsey to repair her due to the way she was created.[28] Throughout the game, Cortana's personality continued to fragment and this was used against her and John-117 by Captain Andrew Del Rio, though John-117 remained confident in her.[30] Cortana was eventually able to take advantage of the fragmentation of her personality while on the Ur-Didact's Mantle's Approach.[33]

She, along with Mendicant Bias, is one of only two known AIs that may have progressed completely through rampancy and became metastable. After most of her was destroyed aboard the Mantle's Approach,[33] the remaining fragments were sent to Genesis, curing her of her rampancy.[39]

As Cortana is an AI, she has no physical body. She can communicate through comm systems and project a holographic image of herself from appropriate projectors, such as Holotanks. Her chosen avatar appears to be a naked female human, roughly in her early twenties, with purplish/bluish skin, shoulder length purplish/bluish hair cut shorter at the back, and blue symbols scrolling down her body. Cortana is said to resemble her creator, Dr. Catherine Halsey as a young woman, with a similar attitude "only unchecked by military and social protocol."[36] Dr. Halsey says that Cortana looks somewhat like Miranda Keyes.[1] By The Reclamation, Cortana had created a physical body for herself. She changed her face as well and her body appears as though she were wearing a circuitry-laced skin-tight suit with layered padding.[4]

When in a different emotional state, Cortana usually changes colors. This was especially noticeable in Halo: Combat Evolved, but less so in Halo 2 and Halo 3. Only in specific instances did she produce the color-changing effect. One instance of this was right before John-117 rescued her.

Bungie took a fair of amount of artistic freedom in the evolution of her appearance throughout the Halo trilogy, noticeably from Halo: Combat Evolved to Halo 2. She had longer hair, a more-realistic humanoid figure, a much more feminine appearance, and a slightly different shade of purple. In Halo 3, she became slightly bluer and again her face, hair, and body were changed to look more feminine and realistic. In Halo: Combat Evolved Anniversary, she has her Halo 3 appearance. In Halo 4, she had darker, more opaque and realistic hair. Her facial structure had noticeably changed, her figure had become considerably more curvacious, and she gained better defined fingers and toes. The circuit-pattern mapping over her body changes between every game as well.

Cortana was designed to infiltrate computer systems and she is excellent at the task. She has become so intelligent that she understands Covenant technology far more than her makers and in some cases, the Covenant themselves. As of early 2553, she has never encountered a Covenant security measure that she wasn't able to bypass. In addition, she is proficient with UNSC military hardware and can run military ships and stations by herself. Indeed, the UNSC's primary use of AIs is to oversee targeting of point-defense weaponry used in space battles. For Cortana, this is merely an additional capability. She has helped out in several major battles including Operation: FIRST STRIKE. She was capable of storing large amounts of data within her holographic appearance, hiding it in the microscopic lines of code that made up her visual form. Even when she was in the Data Crystal Chip, microscopic lines of code are running through the holographic "filling."

Cortana can be transferred among various computer systems. She needed a Data Crystal Chip to move in and out of various networks, including John-117's MJOLNIR armor.[41] However, in one instance, she merely needed to be touching another system to access it; however, this is most likely a product of John-117's upgraded armor.[16] Quite strangely, she reverted to the data chip; this was probably because John-117's armor was badly damaged during the fall to Earth that it reverted to a back-up system, which also explains its different shield.[7] In another instance, instead of having to be physically "plugged in" to the hologram projector, John can just hold the chip close to it. She then "walked" off it, later "touching" the chip to return to it—holographic representations of the data transfer.[23]

One of Cortana's other abilities was that she could control a variety of starships and stations and act as their AI, even though that wasn't her primary purpose. Cortana has acted as the AI of the Pillar of Autumn, Cairo Station, the Forward Unto Dawn, the Gettysburg and even the Covenant flagship Ascendant Justice. Despite it not being her purpose to act as a shipboard AI, Cortana was extremely good at it, with Captain Keyes being so pleased with her performance during the Fall of Reach, he ends up being glad he never got to initialize the Pillar of Autumn's true AI. During the time the Ascendant Justice was under human command, Cortana was able to control that ship easily once she destroyed the Covenant AI and was given full control of its systems by John-117. She proved even more adept at using the ship's technology than the Covenant were and was able to perform feats with it the Covenant never even thought of trying. While she was in control of that ship, Cortana was also able to act as the Gettysburg's AI at the same time, albeit while the ships are linked and is able to transfer herself from one ship to the other at will.

Another of her abilities was the ability to clone herself. She got this from a Covenant AI she destroyed and dissected. She first tested it by copying her language translation abilities and found it worked great. She also could seem to sense what progress the clone had made as if she retained a connection to it. Later, due to the importance of her getting to Earth due to all of the data she'd collected on Halo, the Flood and Covenant technology, she was forbidden from going on John-117's mission, but she was able to make a clone of her infiltration abilities to give John enough of her capabilities to infiltrate the Covenant command and control station. She claimed that this would not be a complete copy, but she thought that what it was would be enough. The clone had at least some of her personality as well. The clone retained her abilities to hack systems and some limited mission planning routines that allowed her to make basic plans to help the Spartans, but was unable to make the complicated plans that Cortana could come up with. The original clone made a copy of itself into the station's systems to help out and, due to being overwhelmed by Covenant security AIs, ended up making hundreds of copies of itself which overwhelmed the system and made accomplishing anything hard for the clones as they began to lose their coherent state, but still could control the system. The original clone was left behind in the reactor system. It was placed there to overload the reactors and was never removed once it started the process. All of the clones created on the mission were destroyed when the station blew, but provided invaluable help to the Spartan team. Later, while held captive by the Gravemind, Cortana used her cloning ability to create data-stripped clones of herself and ones of her lower-priority functions to distract the Gravemind. These were all consumed by the Gravemind, but they helped buy Cortana time and one also helped her create her message to John-117. Some were even responsible for the more rampant parts of two of the Cortana Moments. During Halo 4, while on The Didact's ship, she creates multiple clones of herself through Rampancy to help overwhelm the ship's defenses and lower the Composers shields for John to plant the HAVOK nuclear mine to destroy it with. Cortana and her clones later manifest themselves as hard light to aid John in battle by tying down The Didact so he could plant a Forerunner Grenade to his chest to stun him so he would fall into the event horizon of the Composers energy network.

She was also very good at planning missions for John, including the destruction of Halo. In Halo: The Flood, John comments that he's not sure who's better at blowing things up: him or her. It was also her plan that led to the destruction of all of the Flood that had escaped Installation 05 and saved the galaxy from a further Flood invasion. Apparently, she has mission-planning routines that help her with this as her clone told John that a copy had only limited-mission-planning routines and was only able to help the Spartans form basic plans.

Another ability that she developed was a limited, teleportation ability on Installation 04. She learned how to teleport on Halo while in its control systems, but as she couldn't access Halo's power grid, she couldn't perform this task very often. She did teleport once with John-117 by drawing power from his fusion pack, saying that she was sure it wouldn't cause permanent damage to his power pack or shields, but both agreed that this should only be done once. She learned where they needed to go from the Covenant Battlenet and teleported them there although they ended up upside down due to her not getting the coordinates completely right. She has not used this ability since although she could presumably use it on any Halo but would have the same power requirements problem.

Cortana rarely appears physically in gameplay. She mostly acts as a disembodied voice or appears in cutscenes. She does physically appear in gameplay during the levels Gravemind and High Charity in Halo 2. Here, she will appear on pedestals around the city and will explain what to do. In the Halo 2: Anniversary version of the level, she is depicted in a slightly modified version of her Halo 4 model. In Halo 3, there are several times where she will attempt to communicate with John (and the Arbiter during Co-Op). These moments are called Cortana Moments. In Halo 4, she will occasionally need to be inserted into various terminals. In the Halo 5: Guardians level Reunion, she will highlight various interactable terminals with Oly Oly Oxen Free. Cortana's voice is available in Halo: Reach's Firefight mode.[42] Once she is acquired after the level The Package, her casing will be added to the back of player one in The Pillar of Autumn.

In Halo 4, Cortana's face was modeled after international model Paisha Coffey and her body, after Frances Ingraham and Katrina Le. Her movement was performed by Mackenzie Mason using motion capture technology.[45]

Easter eggs involving Cortana can be found in a couple places. In Halo 3, look for a UNSC mobile computer. Watch the lower-left monitor. After a few seconds, Cortana's face will appear for an instant and then disappear. Two of the easiest ways to see this is to either look at the aforementioned screen in the beginning of Crow's Nest, or at one of the computers in the multiplayer map High Ground.

There was going to be an alternate story in Halo: Combat Evolved in which John-117 rediscovered Cortana inside the control room and she had become mad with power, wanted to take over Installation 04 and ultimately, the universe.[46]

In the original draft of the Halo: Combat Evolved script, Cortana was supposed to have an accent akin to the British Isles. When in search of voice actors, Martin O'Donnell made sure they had the ability to interpret the accent correctly.[46] This is evidenced by the fact that in Halo: Combat Evolved, Cortana uses several British idioms, such as "Sod off!" when addressing 343 Guilty Spark.[47]

She was rated as one of the ten most disturbingly sexual game characters by Games.net and one of the fifty greatest female video game characters ever by Tom's Games.[48]

In Halo 3, she repeated lines Dr. Halsey has said before such as "You will be the protector of Earth and all her colonies." which Dr. Halsey says to the 6- year- old Spartans in Halo: The Fall of Reach. Cortana also recites a few lines from the Cortana Letters in Halo 3 in some of her messages and in the announcement trailer. Some of the lines are said in the Cortana moments in the level Cortana. At certain points in the Halo 4 campaign, Cortana during moments of rampancy is identified on John-117's HUD as Dr. Halsey and often repeats Halsey's quotes.

Cortana had many similarities to the Marathon AI, Leela, such as control over certain functions of ships and other locations in their computer systems.

Being a smart AI, Cortana's operational life span was approximately seven years. However, after she took in all the Forerunner information in Halo's control room, her estimated life span was possibly halved, due to the vast task of storing, processing etc. all that information.[49] In addition, all of the data she absorbed on Halo noticeably decreased her efficiency.[49]

The name Cortana (also Curtana or Courtain) is a Latinized form of the Anglo-French curtein, from Latin curtus meaning "shortened." The name is used for a ceremonial type of sword. According to legend, one such weapon was the sword of Ogier the Dane - it bore the inscription "My name is Cortana, of the same steel and temper as Joyeuse and Durendal." Durendal (spelled 'Durandal' in this sense) was the name of an Artificial intelligence in a previous Bungie game, Marathon. This also ties in with Cortana telling John "I am your shield, I am your sword."

In Halo: Glasslands, The smart AI "BB" sees Cortana as a negative role model. He believes she has become far too human after spending so much time around them.

In Halo: Combat Evolved Anniversary, Cortana takes her Halo 3 form. In the game, Cortana always remains in a blue color, unlike the original, where she is seen in a purple color, and changes to a green color when in Installation 04's control center.

On the level Requiem, an audio log can be found in which Dr. Halsey mentions how Cortana could possibly outlive the 7-year life expectancy before saying how odd it is for a UNSC AI. While she did outlast the 7-year period, she performed poorly afterwards.

Jen Taylor was awarded "Performance of the Year (Female)" by Ben of "TheGamerDrive," for her performance in Halo 4.[50]

The Didact seemed to show some interest in Cortana, perhaps because of how advanced Cortana was considering humanity and its lower than expected technological sophistication.

Cortana was chosen to be the name for the personal assistant in Windows Phone 8.1 and Windows 10.[51] The name was originally "a code name to rally the team around what they were building," but the decision to keep the name was made as a result of petitions and polls asking for the name to be kept when the code name was leaked.[52][53]